Frequency eliminator



March 3, 1931. GREEN 1,794,847

FREQUENCY ELIMINATOR Filed March 18, 1927 5% iger yr 3 Freyuemcy J vINVENTOR. 1:1 1:1 :3

El Gneen/ v ME 14/. Know/E Patented Mar. 3,1931

venom: AND TELEGRAPH COMPANY, ACORPORATIQN OFNEW' YORK Apication"iiieafiuareh'is, 1 927.l"se1 i'ai lief 176,545. :1 I

' This invention relates to signaling systems, 7 andparticularly toarrangements in such sy stems for elim nating currents of defin tefrequencies.

carrier frequency corresponding to the voice frequency signals, it isfrequently thepractice to employ'an electrical wave filter to sup-gpress one ,of the side bands A and to freely the frequency spectrum, hasbeen extremelydifiicult of accomplishment with ,theordin'ary designs ofelectrical wave filters; I any of the component frequencies of theundesired 7 side band are transmitted through theeleetrical wave filter,such transmitted frequencies will become demodulated at the receiv ingterminal of thesystem, andthereby constitute a source ofinterference,the seriousness of whichwill .depend largely upon the magnitudes of suchtransmitted frequencies and upon the phase relationship existent be-vtween the current of carrier frequency at the receiving terminal and thecurrent'of car.-

rier frequency at the transmitting terminal.

Accordingly, it is an object of thisinvention to employ a p1ezo-electr1cstructure to act as a means'of suppressing undes rable frequencies at ornear the cut-0E pointofan 7 electrical wave filter.

It is'another object of this invention to as sociate a piezo-electricstructurewith, a ea frequency spectrum. a

vice exhibiting attenuation which is variable with frequency, thepieZo-electric structure being operated so as to provide very highattenuation for a number of frequencies or for all frequencies within asmall region of the While this invention will be pointed out withparticularity in the appended claims, the invention itself, both as toits further objects and features, will be better understood from thedetailed description hereinafter following, when read in connection withthe In systems in which current of a carrier frequency is modulated byvoiceffrequencyf signals so as to produce sidebands of the 'nsrrtii I.ennnn,, or EAST ORANGE, newuneau, assienoia ro-AMEnIci-in "rain--accompanying drawing, which Figure l the inventionlv V Referringto Fig lof the drawing, there is shown a source o'f signals S connected to' theprimaryfwinding of the transformer T The secondary windings of thetransformer i represents a signaling'system embodying the a princ1plesj;of this invention, Fig; 31a repre- 1 sents aportlon of the signalingsystem shown sents curves characterist c of the elementsof T areconnected to a duplex modulating system having two three-elementthermionic tubes V and V the filaments of which are heated toelectron-emitting temperatures by a commonbattery. Thesethermionic tubesare q preferably similar and equal in their fstrucfturealndcharacteristics, and'maybe'replaced by "a single d uplex tube havinga'single 'filament, two. grids and twoplates] The input circuits ofthese thermionic tubes are arrangedin parallel with respect-to thesecondary winding oflthe transformerT which "winding suuseries withabatteryB n the: conductor" common to the u-mput circuits of these"tubes; The output I circuits of these tubes" are "connected through theprimary windings ofa transformer T in parallel with respect to aconimonplate battery B The r i I J connected to an electrical wave filterwhich. -may be of any well-known type, preferably secondary winding ofthe transformer T is ofthe type disclosed inthe patent me. A.

' Campbell, No. 1,227,113, dated May22,1917.

The output terminals 'of the filter F are connected tothe primarywinding of the The secondary winding of the transformer T is connectedtothe input terminals of, a translating device, such as a three-elementthermionic tubeV,jthe input transformerfT circuit of which includes thegrid and filament of the thermionic tube V, a battery B and thesecondary winding of the transformer T4. The filament of the thermionictube -V also heated to an electron emitting temperature by alocalbattery, whichmay, in this illus tration, be the same as the batteryheats I the filaments of the thermionic tubes V and I V toelectron-emitting, temperatures -The output circuit of the thermionictube'V in-' T and a battery B The secondary winding of the transformer Tis connected to an out- A piezo-electric device PE is type, is connectedto the primary windingof J n V l V 'irequencies at or near the cut-offpoint of the put circuit S eludes the filament and plate of thethermionic tube V, the primaryjwinding of transformer connectedinshuntwiththe input circuit of the thermionic tube V, the piezoelectricde-' 'vice being shown brldglng the secondary w nding of the transformerT known type, preferably of the vacuum tube the transformer T Thisoscillator generr ates current of the carrier frequency to be employedin the process of modulation in the system. By 'virtueof the connectionof the up "oscillator O and the transformer T inthe duleX translatin sstem the rids ofthe tubes 7 V andV are both in phase with, respect tocurrent oftheca'rrier frequency transmitted by this-oscillator.Oscillations of the'carr e'r frequency are thus impressed in the samedirection on the input circuits of these tubes, the grids usuallybecoming equally biased'negatively by the battery B Under ordinaryconditions, equal currents of the carrier frequency willfiow intheoutputcircuits of the circuits ofthe tubes V and V ,modulate curtubes V and Vwhich'may be oppositely directed by the primary windings of thetransformer T so that these currents may elfectively balance andneutralize each other.

source S when impressed upon the input theoscillator O, sothat sidebands of the carrier frequency, known as the upper and lower side bands,may appear in the out ut circuits of these tubes and may be transmittedthrough transformer T to the filter 'F, current of the carrier frequencybeing effectively suppressed by virtue'of the balanced arrangement ofthe I duplex system. This electrical wave filter F rmay, of course, beeither a high-pass filter, a low-passfilter or a band-pass filter. IYet, for illustrative purposes, it may be assumed that this filter is ofthe band-pass type, freely transmitting one ofthe side hands resultingfrom modulation, and substantially suppressing the other side hand. Forillustrative purposes, it may also be assumed that the electrical Wavefilter F substantially suppresses the lowerside band, while freelytransmitting the upper side band. .In systems similar to the oneillustrated, it has been discovered that if any, of the frequencies ofthe undesired side band, i. e., the lower side band, are transmittedthrough the filter F, the frequencies of the undesired side band, whichare thus transmitted, will become demodulated at the receiving terminalof the system when beaten 1 with current of the-carrier frequency whichis supplied at that terminal. The presence of the frequencies withinthe'undesired side band will constitute a source of interferenceAn'oscillatorO, which may be wellslgnaling "currents; orig nating 1n thetending to distort thesignals. The eflect of these transmittedfrequencies in causing. in-

terference will depend largely upon the amplitudes ofthese transmittedfrequencies and upon the degree of synchronism betweenthe system.

c c r 10 carr er. currents supplied at the transm tting terminal and atthe receiving'terminal of the v The piezoee-lectric device PE isdesigned tq have a resonant perio'd such that itmay 7 :present in. theinput circuit ofthe thermionic tube Vaipath of very low impedance for Velectrical wave filter Fr lVhen viewed from.

a different angle, the piezo-electric device may be cons dered one whchexhlb ts very ly cqual to the period of the current of carrierfrequency. In other Words, the vibratory period of the piezoelectricdevice may be the same, oralmost the same, as the period of the currentofjcarrier frequency. At frequencies very close to the natural frequencyof the piezoelectric deviceiPE, there will be high attenuations, these'attenuations additively increasing the attenuations of the electricalwave filter F narrow range. I y

In Fig. 2 of the drawing, qbcrepresents a curve characteristic ofan'electrical wave filter of the band-pass type, in which the ordinatesrepresent loss orflattenuation and the abscissa represent frequency.This curveindicates that the ele'ctricalwave filter exhibits a low. andsubstantially uniform attenuation within a range of frequencies somewhatwell-defined in the frequency spec trum. The curve def represents acurve characteristic of a pieZo-electric device, from. whlch 1t w ll beapparent that there 1s:very

high attenuation for each frequency within a small band also somewhatwell-defined in the freql'iency spectrum. It will beapparent from thedrawing that the band-pass filter for all frequencies within a willtransmit freely and uniformly all frequencies within a desired side bandand effectively suppress frequencies within the unde sired side'bandvery remote from the frequency of the carr er current. The niagnitudesof the loss or attenuation presented by the filter 1n the range of theundesired side band which is immediately adjacent to the frequency ofthecarrier current, may, however, be insuflicient to prevent interferenceand distortion of the signals if some of thefrequencies in the:undesired side band do become transmitted to the receiving terminal.The-employment of a piezo-electrical crystal designed to have a suitablevibratory period 2O I tory periods areshown connected in shunt will,however, provide a material improve ment in the suppression offrequencies of the undesired side band closely adjacent to the frequencyof the carrier current. The curve clef shows the amountof'attenuation1ntro *duced by thepiezo-electric device, and'shows alsohow important such a device may be for providing additional attenuationin the range of frequencies at which the filter provides insufficientattenuation. a

It will be obvious that if the band of fire quencies intended forsuppression in accord once with the principles of this inventiomis toowide for elimination with a single piezoelectric device, two or moreofsuch piezoelectric devices may be suitably combined to providesuflicient attenuation for all fre V quencies within that band. In theillustrated system of Fig. 1a,. additional piezo-electric devices PEa-nd'PE having different vibracially one which may exhibit attenuationwhich is variable with frequency.

While this lnvention has been shown and described 1n one particularembodiment,

; merely for the purpose of illustration, the

general principles of this invention maybe applied to other and w delyvaried organizations without departing from the spirit of the inventionand the scope of the appended claims, r J

What is claimed is: 1. The combination of an electrical wave filterhaving a flat transmission characteris tie for frequencies above apredetermined frequency and a variable attenuation characteristic forfrequencies below said predetermined frequency, and a piezo-electricdevice coupled to said electrical wave filterrhaving a natural frequencyof vibration adjacent to said predetermined frequency.

2. The combination of an electrical wave filter having an attenuationcharacteristic which varies with frequency immediately adjacent to thecut-off point of said filter, and a piezo-electric structure having avibrato frequency close to the cut-off point of sai filter, and meanswhereby said PTGZO'. electric structure may exhibit a high attenuationfor all frequencies within a narrow region of the frequency spectrumadjacent to its vibratory frequency.

3. The combination of an electrical wave filter having a cut-off pointwell defined in the frequency spectrum, and means coupled to saidelectrical wave filter including a piezo-electric structure having avibratory period such that it provides high. attenuation for a range offrequencies immediately adjacent to the cut-ofi' point of said electri-l' cal wavefilter.

4. In a signaling system thes combination of a source of high frequencycurrent, is sourceof signals, means for beating the current of saidhigh. frequencysource with the current of said' s'ignaling- 'source'soa's to produce two side ban'ds ofv the high frequency V current reachcharacteristic of said signals, an electrical-wave filter for freelytransmit tingnone ofithe side bands and for substan suppressing theother of the-side? of a source of signals, a source of'carrier current,a duplex translating circuit for beating the carrier current with saidsignals in order to provide a pair of side bands of the carrier currenteach characteristic of the signals, an electrical wave filter forsuppressing one of the side bands and'for trans mitting the otherof theside bands, and-"a piezoelectric structure connected to said electricalwave filter which may be vibrated at a frequency substantially equal tothe cut-off frequency of said wave filter adja- 7 cent toth'e1 frequencyof the carrierv current, said piezo-electric structure sharpening thecut-off of said electrical wave filter at the frequency or frequenciesat which'it is vibrated.

6. The combination of an electrical wave filter having a substantially.uniform transmission characteristic over a band of frequencies andhavinga non-uniform attenuation characteristic for a range. offrequencies immediately adjacent to said band, and piezoelectricmeansjconnected to said electrical wave filter to substantially increasethe attenuation characteristic of said filter for all frequencies withina narrow band im- {)nediiately adjacentfto the first-mentioned- 7. Thecombination of a band-pass filter and a piezo-electric device coupled tosaid filter having a frequency of vibration. substantially equal tooneof the cut-off frequencies of said filter, 7

"8. A band filter comprising a plurality of piezo-electric devices ofdifferent vibratory periods connected in parallel relationship, thevibratory period of each device being within the limits'of the band,each piezo-electric device suppressing currents of frequencies within anarrow portion of said band, the 1 ious piezo-electric devices beingadjacent to each other. I i

. 9. A band filter comprising a plurality'of portions. of said bandsuppressed'by the var-.

piezo-electric devices connected in parallel relationship and havingclosely spaced nat-' ural vibratory frequencies so thatthe Variouspiezot-electric devices may suppress cur-- rents having frequenciesWithin adjacent portions of the band to be eliminated.

10. The combination, of a band-pass filter and apluralityofpiezo-electric devices cou-" pled to said filter havingdifi'ierent vibratory frequencies, the vibratory frequency ogf, eachdevicev being ar a selected cut-off frequency' of said filter.

In testimony whereof, I have signed my name to this specification this17th day of 15 March,

ESTI-LL I. GREEN.

